Who shouldn't use it: There are two categories of risk with permanent makeup: medical and cosmetic. As with any tattoo, there is the risk of blood infections (including HIV and hepatitis) from poor needle hygiene.
Keloid scars and inflamed tissue called granulomas may grow around injection sites. Some people have an allergic reaction to the tattoo ink, which is problematic because the pigment must then be removed. Most strangely, tattoos around the eye area can interfere with MRIs.
There is also the risk of unpleasant cosmetic results, ranging from tattooed eyebrows that are too close together, lip pigment that turns out to be the wrong shade, too-thick permanent eyeliner, and so on. In these cases, the unhappy client has the option of either waiting for the makeup to fade or turning to laser tattoo removal, which is painful and costly.